Open minded advice and knowledge that surpasses any guitar amp forum I have followed. I really wish I could put together the proper praise you guys deserve in words!

I have built about 15 tube amp projects in the past year(mostly guitar amp clones), Some worked great! And others, Well, Ahem, nice frequency generator/oscillators LOL.

I love my little RH84 SE and still use it daily on my laptop!!

Now I want to try to really move a step forward into the higher power ranges. I am thinking PP 6550's running about 50-60W range, Possibly more but 50-60w is ballpark and should not push the tubes to badly.

Getting the Right Starter parts,
#1, Iron.
After doing a lot of searches both here and elsewhere, It looks like the OPT of choice will be Hammond 1650N 60W 4,300 ct 318 ma. 4-8-16 selectable output
PT, This is the one that seems to bit me every time, I buy nearly 95% of the time, To big of a unit.

So, My question is basically how to select or calculate easily the size transformer required?
I had This unit in mind.
378X: 400-0-400/200ma./5.0vct@3amp/6.3vct@6amp/50v bias tap.

#2 Chassis material.
All of my projects have been built into Hammond Steel Chassis*.
This project should (I hope) fit into or onto a 17" x 10" x 3" which fits my current head cabinet I plan to recycle.

Hammond sells steel or aluminum at .04 inches thick.
*I have read on several guitar amp forums that steel is a No No in guitar amps? Claims of oscillation issues do to magnetic coupling ect?
I am not sure the aluminum can handle that much weight.

I plan on photo documenting this project and sharing it with whomever wishes to have the info on the completed unit. But I want to make this unit reliable, duplicatable and most importantly Sound Great.

I would love some input.
Thanks
Trout

testlab

11th March 2006 12:25 AM

For an output transformer, see if you can find an old Leslie amp chassis from a 122/147.

As for the steel chassis no-no, the coupling is due to transformer orientation, not the chassis. Make sure the output and power laminations are turned 90 degrees to each other.

Trout

11th March 2006 01:11 AM

Quote:

Originally posted by testlab For an output transformer, see if you can find an old Leslie amp chassis from a 122/147.

As for the steel chassis no-no, the coupling is due to transformer orientation, not the chassis. Make sure the output and power laminations are turned 90 degrees to each other.

The orientation thing is clearly one valid point, I have never had audible hum issues so I think the current layout seems fine for this. I had read more concerns about possible ground looping and parasitic oscillations with Steel Vs Aluminum. Though I am aware there are several other variables that apply, I want to go ground up and truly get it as near perfection as possibly. So each part needs to have maximum scrutiny.

The old/vintage output transformer idea is a good cost measure, But I am looking to build with current production fairly easy to acquire items, Hammond of course being high on the list for that reason. There are of course several other brands of transformers at varying prices, But my principle concern at this point is getting them at the correct matching specs for optimum performance.

My plans are to build no less than 4 units, My younger brothers Band has progressed to the point of opening act for a rather well known 80's rock band this summer, And these units are going to see considerable gig use.

He has been using a couple of Fender 5E7 30W clone types I have built, Loves the Tones but, Just plain needs verb in power.

I/We do not really wish to use cloned products at this point, Though its a fact its all ancient science that has easily proved to be superior to any silicon product.
The price tags on current high quality valve guitar amps, The ones that actually excel in sound quality make them out of the ball park in cost effectiveness. Especially in a here today gone tomorrow band scenario.
Trout

lndm

11th March 2006 02:02 AM

An iron chassis actually makes for a magnetic shield, but still heed the advice you have received elsewhere.

318mA. That is kinda large. The thing is, doing push pull the DC from the two triodes is flowing in an opposing manner and cancels each other as far as the magnetism is concerned (ie no DC).

A PP xfmr can have a closed magnetic structure - that is, a SE xfmr has to have separate E and I laminations with a gap in between, because otherwise the iron would saturate from the DC.

This means the PP xfmr is more magnetically efficient, there is more inductance from the windings, the size only needs to be as big as needed for the power differential (speaker output). PP xfmrs are smaller, even though they deal with higher output.

Dave Cigna

11th March 2006 01:03 PM

Concerning the output transformer, The limiting factor is the lowest frequency you need to produce. Low E on a guitar is roughly 80Hz, which is 2 octaves higher than the 20Hz that most playback systems are (supposedly) speced at. What that means is that you can use an OT much smaller than you would need for a 50W stereo amp. That's the real reason that the OT's in guitar amps are small compared to those in stereos. It's also the reason that OT's in bass amps are larger than those in guitar amps. Leo Fender did not design guitar amps to distort. He designed them to sound heavenly when played clean. The fact that they sound good when turned up is a side-effect of the good clean sound. At least that's my belief.

Having said all that, bass amps do seem to have tighter bottom end than guitar amps. If you want that then a larger (than usual guitar amp) OT would be a good choice. My point is that you really don't need to worry about under specing it. The Hammond unit should be more than enough. (I have an old Traynor Bass Master with a huge Hammond OT and I just love the tight ballsy tone. I've bought and sold lots of old amps, but I'll never let that one go.)

As for chassis material, I think that steel is not a problem unless you use it as signal ground. Use a separate (large) ground buss wire and it will not give you any trouble. If the amps will be used for gigs then the extra ruggedness of steel will be important, I think.

I think the power supply in guitar amps is where some of the playability comes from. I've found that if it's over done and too stiff then some of the touch sensitivity is lost. It's easy these days to add a ton of capacitance, but I find that it robs the tone. An over size PT is not quite as bad, though I do recommend tube rectification in any case. I suppose it depends on what kind of sound you're looking for.

To address your question more directly, I would look for something around 350V @ 300mA. I don't think Hammond makes anything like that (why not?) An ST70 replacement PT might be a good choice. I've heard that the ones at Triode Electronics are good.

amperex

11th March 2006 02:57 PM

Check out Weber Speaker for inexpensive power & audio transformers.

Trout

13th March 2006 03:25 PM

Looking around at various PTs, Finding something to push EL34's and or 6550's is a mixed bag.

Seeing as we are looking to build units in the 50-60W output range, The Marshall replacements look promising, And the only drawback I see to those are #1, I can find no actual specs on the opt's no matter where I look, Like its some guarded secret, And The only PTs I see use solid state rects, Not exactly as I hoped.

I had been looking over a Hammond Compatability list at The London Power website, And They have a couple of possible matches lower on their page.

Concerning the output transformer, The limiting factor is the lowest frequency you need to produce. Low E on a guitar is roughly 80Hz, which is 2 octaves higher than the 20Hz that most playback systems are (supposedly) speced at. What that means is that you can use an OT much smaller than you would need for a 50W stereo amp.

I had found back in the 70's when I was young and could still hear, I had an 300w Ampeg SVT all valve bass amp that made the sweetest lead amp I ever heard, At least for the music we played back then.
Bass amps just plain sound sweet with a guitar in most cases because the bottom end tightens up so much. I have never found a lack oh highs, So that Bassman replacement MIGHT be what I am looking for.

If I bought the matching PT it would be a real sore spot, I want to run a tube rect and the matching PT below requires SS.

This is where I generally mess up, I 99% of the time buy PT's at 30% or more over on the ma rateing.
How low is not enough, and how much is to much?
Or am I just asking for trouble using Tube Rect Vs SS once I hit this level?

Trout

poobah

13th March 2006 05:02 PM

Hey Gene,

Depending on what actual schematic you use you may not need the bias tap.

You probably want a little more B+ for 6550's... say 410-420.

As far tube versus SS rectification is concerned... you CAN put resistors in series with ss diodes to mimic the effect quite well. This is cool because it simply gets rid a of tube (tubes are cool... but we all know ss has em whupped). This does put B+ on cold tubes... but these ARE guitar amps we're talking about; the tubes will get pounded anyway (we hope).

Stay light on the power trans if possible... a good HiFi supply is a bad guitar amp supply. I can weigh my Leslie trans for you if that would help...